This invention relates in general to a voltage regulator circuit, and, more particularly, relates to a breakdown voltage protection circuit for limiting the voltage applied to a junction to a non-destructive value.
In the operation of a MOS circuit, overvoltages are sometimes inadvertently applied to the circuit. The circuit must be protected from these overvoltages to prevent degradation or destruction of the circuit.
Like any semiconductor junctions, the drain substrate or source substrate junctions of a MOS transistor are characterized by a breakdown voltage when reverse biased. At reverse voltages in excess of this breakdown voltage the critical field is exceeded, the junction is no longer rectifying, and the reverse current increases rapidly. In a MOS transistor the junction breakdown is very heavily influenced, however, by the presence of and bias on a gate electrode located in proximity to the junction. It is well known, for example, that for a MOS transistor with the gate at a lower voltage than the drain voltage (e.g. gate grounded) the drain substrate breakdown is significantly lower than that of a similar but ungated junction. The negative gate-to-drain bias (for N-channel transistors) inverts the weakly doped periphery or fringe of the drain junction, especially at the semiconductor surface where the electrostatic field is greatest. The gate bias causes a reduced radius of curvature of the drain junction at the surface and an increased doping gradient of the diffused drain region which in turn reduces the depletion width within the drain region. The net result is a lowering of the junction breakdown voltage because of resulting higher field strength.
An N-channel MOS transistor can also be viewed as a lateral NPN bipolar transistor in which the source becomes the emitter, the substrate becomes the base, and the drain becomes the collector. If the electric field at any point on the drain substrate junction exceeds the critical field, an avalanche breakdown of the junction occurs with a resulting current flow from drain to substrate. This substrate current causes a localized voltage drop in the substrate which can have a significant effect on device operation. If the localized voltage drop in the substrate is sufficient to forward bias the base emitter (substrate source) junction, the lateral NPN transistor is turned on. This results in a drastic increase in drain current and an associated drop in sustaining drain-to-source voltage. This current voltage characteristic is equivalent to the "switchback" characteristic of a bipolar transistor operated in the CEO mode. The excess drain current associated with the switchback of the lateral NPN transistor does not flow into the substrate, but instead flows into the source. That excess of drain current caused by the turn on of the lateral NPN transistor can cause a permanent failure of the device by locally heating and fusing together the source and drain junctions.
High voltages, that is, voltages in excess of the junction breakdown voltage and especially the breakdown voltage of the junction as modulated by the presence of an adversely biased gate electrode, are occasionally applied to a semiconductor circuit. This occasion is especially likely in such circuits as electrically programmable read-only memories (EPROM) which require voltages in the range of 25 volts or more for programming. In view of the possibility of such high voltages being applied to a semiconductor circuit, it is necessary to protect such circuits from damage or permanent failure which can be caused when breakdown voltages are exceeded.
It is therefore an object of this invention to provide a voltage regulating circuit which limits voltages to values which will not cause destruction of a circuit.
It is a further object of this invention to provide a voltage regulating circuit which provides high voltage junction breakdown protection having a desirable loadline feature.
It is another object of this invention to provide a self tracking breakdown protection circuit which allows application to a circuit of the maximum possible nondestructive voltage.
It is still another object of this invention to provide a voltage regulating circuit which tracks with changes in breakdown voltage of circuit devices.